add contributing document (#167)

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# how to contribute to NGM
+this page is a very simple document with some notions about NGM and things to
+follow when willing to contribute.
+nothing too fancy, just to set the bar of the project :wink:
+
+## structure of the project
+NGM ships two Nushell packages
+- `nu-git-manager`: the main package that defines the `gm` command and most
+  important subcommands
+```
+pkgs/
+`-- nu-git-manager
+    |-- nu-git-manager
+    |   |   ...              # any internal or public module should go there
+    |   `-- mod.nu
+    |-- nupm.nuon
+    `-- tests
+        |   ...              # test modules should go there
+        `-- mod.nu
+```
+- `nu-git-manager-sugar`: an optional package that ships additional `gm`
+  subcommands or other Git-related modules
+```
+pkgs/
+`-- nu-git-manager-sugar
+    |-- nu-git-manager-sugar
+    |   |   ...              # any internal or public module should go there
+    |   `-- mod.nu
+    |-- nupm.nuon
+    `-- tests
+        |   ...              # test modules should go there
+        `-- mod.nu
+```
+
+## conventions
+- all commands should be named as a subcommand of `gm`, e.g. `gm foo bar my-super-cmd`
+- command names should be put inside quotes (`"`), e.g. `def "foo"` instead of `def foo`